Ammonia selective sensors based on cobalt spinel prepared by combustion synthesis

Nano-crystalline cobalt spinel was prepared by combustion synthesis and used as ammonia sensing material. After synthesis, the powder was calcined at 600 °C for 4 h and characterized by thermal analysis, X-ray diffraction, Raman spectroscopy, X-ray Photoelectron Spectroscopy, nitrogen adsorption (B.E.T, Brunauer, Emmet, Teller and B.J.H., Barrett, Joyner, Halenda techniques), H2 temperature programmed reduction, H2O adsorption and field emission scanning electron microscopy. Sensors were screen-printed onto α-alumina substrates with platinum interdigitated electrodes and fired at 700 °C for 1 h in air, after drying overnight. The sensor response was measured in the range 150 °C–250 °C under 1–50 ppm of NH3. Best results were obtained at 225 °C, with R (the ratio between the impedance of the film under gas exposure at the equilibrium and the impedance under dry air) equal to 1.83 under 50 ppm NH3. Response time and recovery time (e.g., the times taken by the sensor to attain 90% of total impedance change from its initial impedance value) were determined, together with cross-sensitivity tests towards CH4, CO, N2O, humidity, O3, CO2 and NO2 at the best operating temperature.